Tags

A customer has a Safety Instrumented Function they need to implement with two independent level transmitters (radar), each wired to a separate STA Functional Safety Trip Alarm. The process will trip if either level reaches the set point, for example a 1 out of 2 (1oo2) voting. However, if the alarm trip (STA) detects an input or unit fault. The customer did not want the process to trip. Instead they wanted to take the faulty unit out of service leaving the process running with the one good level and alarm trip.

Problem: When sharing a Process Variable (PV) with multiple systems, the connection needs to be thought out carefully. Sending a signal go to more than one PLC needs to be well thought out just as carefully.

Challenge:Measure inlet and outlet temperature of a process heater using existing three-wire RTDs and provide a 4-20mA process variable signal, representing the differential temperature across the heater, to the Basic Process Control System (BPCS). The measurements are in a hazardous area and plant standards require the use of a certified IS system.

Before converting a dry contact to an analog signal it is important to first determine what signal levels you will need. Since the TMZ accepts a 4-20mA signal then we can choose any current values within 4mA and 20mA. For this application we chose 'NOT FULL' to be equal to 20mA, and “NEAR FULL” to be represented at 4mA.